An installation with portable, cordless telephone sets which utilizes time division multiple access (TDMA) is already described in such as the Swedish patent 85.02319-0. In this known installation there is a plurality of stationary radio units connected to a radio exchange via a wire connection. One or more telephone handsets are in radio communication with each of the radio units, and between the radio units the portable handsets are usable within a short distance from a radio unit.
In installations or systems with TDMA, a base station can have several calls in progress simultaneously, by utilizing several time slots on a single radio frequency within a frame, as is described in the above mentioned patent specification. In the application of cordless office telephony it has been proposed by different manufacturers to have a total of 32 different time slots in a frame, 16 slots being reserved for the transmission direction of the radio unit handset and 16 slots for the reception direction of the handset radio unit, see the accompanying FIG. 1. If only one radio frequency is used, a so-called combiner is avoided, i.e. a unit which, when several radio frequencies are used, multiplexes the time slots associated with each of the radio frequencies.
However, it is not always practicable to allot a single TDMA frame to all call channels which have been assigned a base station. The bit rate and top power is namely proportional to the number of time slots, i.e. the greater the number of slots allotted to a frame, the higher the top power which must be selected from a transmitter in the base station and in the mobile handset. In systems with many channels, and thus many time slots, TDMA is normally used with several carrier frequencies. The accompanying FIG. 2 is a diagram showing the distribution of four carrier frequencies f.sub.1 -f.sub.4. Each frequency is assigned a given bandwidth B1-B4 and each is alloted a TDMA frame with N time slots. This distribution is still cost-saving, since the total number of radio frequencies (and thereby the number of transmitter/receivers) is N times less than in traditional systems using a carrier frequency-channel FDMA. However, a combiner is required once again.
Mobile telephone systems in use at present use a fixed channel allotment, i.e. each base station has access to an array of specified frequency channels, which can all be utilized simultaneously if so required. The allotted frequencies can be used without risk of interference (noise) since the same array of channels is only allotted to base stations with sufficiently separated geographical positions.
A channel allocation used more recently is the so-called dynamic channel allocation, as described in the above mentioned patent, for example. According to this, all base stations in the system have access to all channels, where each channel has one, or is assigned a given radio frequency and a given time slot. When a call is to be connected, the channel is selected which is unoccupied at the moment and which is least affected by noise. This results in that greater traffic reckoned in MHz bandwidth can be offered than is the case with fixed channel allocation. The system thus adapts itself to local traffic top load variation and to possible shadow effect variations. In contradistinction to fixed channel allocation, dynamic channel allocation results in that each base never uses (or is intended to use) all allotted channels simultaneously.